rbd-wnbd: wait for the disk cleanup to complete

rbd-wnbd: wait for the disk cleanup to complete

The WNBD disk removal workflow is asynchronous, which is why we'll
need to wait for the cleanup to complete when stopping the service.

The "disconnect_all_mappings" function is moved to
RbdMappingDispatcher::stop, allowing us to access the mapping list
more easily and reject new mappings after a stop has been requested.

While at it, we'll log service stop requests.

Signed-off-by: Lucian Petrut <lpetrut@cloudbasesolutions.com>